Generally, this invention relates to communication devices which use subscriber identification module (SIM) cards for identification purposes, and more specifically, to a method and apparatus for protecting a SIM card.
In current communication device systems such as the Global System for Mobile Communications (GSM) radiotelephone system currently in use in Europe, a SIM card is inserted into a radiotelephone for providing subscriber identification, billing information and other information concerning the operation of the communication device. In the GSM system, the same hardware can be used with any SIM card inserted into the SIM card reader of the radiotelephone.
SIM cards are susceptible to a latch-up condition that can destroy the information in the card. Latch-up can occur when a SIM card is inappropriately powered. This can cause input diode protection for the SIM card circuitry to latch in a conductive state, drawing excessive current and eventually destroying the circuit and the information therein. Therefore, the radiotelephone must know when it can power the SIM card so as to keep it from latching up and being destroyed. Previous methods to prevent destruction of the SIM card have involved additional hardware in the radiotelephone.
One previous method included a mechanical switch to detect the presence of the SIM card. However, this method is unreliable due to mechanical bounce problems and the requirement of good, repeatable alignment of the SIM card in the radiotelephone.
Another previous method involved placing the SIM card beneath the battery of the radiotelephone so that the SIM card is installed before the battery. In addition, the presence of the battery was detected by checking for the presence of a battery thermistor. This method requires the use of more expensive, custom battery modules having a thermistor instead of off-the-shelf batteries.
A third method of detection involved the use of an electrical switch to disconnect the battery from the B+rail of the radiotelephone when an alternate power source is present. The disadvantage here is the added cost due to not only the switch but also the alternate power source. Further the alternate power source must be regulated to prevent damage the phone. An unregulated power source cannot operate the radiotelephone without risk to the other phone circuits.
It would be of benefit to provide an apparatus and method to detect those times during which it is safe to power the SIM card in a communication device. Further, it is desirable to provide this protection without any additional hardware or cost. Additionally, the acceptance of an unregulated external power source would be beneficial. It would also be useful if the power protection provided to prevent latch up of the SIM could also be used to prevent tampering with the SIM card while the communication device is operating.